This invention relates to a charged particle beam device for the examination of specimens. In particular, this invention relates to a beam column where the beam may land on the specimen surface at an oblique landing angle.
In charged particle beam devices, such as a scanning electron microscope (SEM) the typical aperture angle as well as the typical landing angle of the charged particle beam is of the order of several millirads. For many applications it is desirable that the charged particle beam lands on the sample surface at a much larger angle of about 5xc2x0 to 10xc2x0, corresponding to 90 to 180 millirads.
One application which requires such landing angles is the stereoscopic visualization of a specimen surface. Stereographic techniques using a SEM date back to the early developmental period of scanning electron microscopy. Since electrons can be collected from practically all parts of a relatively rough sample, a SEM image has a rather xe2x80x9crealxe2x80x9d appearance. The main reason for this real appearance is that the secondary electron signal produced at the point of beam impact varies with the local slope of the surface in the same way as the perceived brightness of the surface of a diffusely illuminated macroscopic object. Furthermore, variations in the efficiency with which this signal is collected by the weak electric field from the detector modifies the signal as a function of position such that it appears as if the sample surface contained shadows. While the images have thus all the visual cues of a conventional black and white photograph, these cues are in many situations deceptive. It is therefore essential that a method which provides authentic perspective information is available. Stereoscopic visualization is such a method. It is useful and sometimes indispensable for detecting and resolving situations where other coding mechanisms yield ambiguous results.
In another application, topographical information about the specimen surface may be extracted, for example, from the parallax between stereo pairs of images obtained with a tilted beam. A further application, three-dimensional imaging of a specimen, requires also a beam tilted by several degrees, see, e.g., U.S. Pat. No. 5,734,164.
In all these applications, the beam tilting mechanism plays a key role. In early solutions, a stereo effect was achieved by mechanically tilting the specimen to provide two perspectives. However, due to mechanical imperfections, a lateral movement of the specimen is inevitable, which often results in misregistrations between the elements of a stereo image pair. This problem is especially pertinent for highly regular structures such as an array of memory cells in an integrated circuit.
When beam tilting is carried out electrically, the fact that the specimen can remain horizontal is a significant advantage as far as the lateral coordinate registration is concerned. Electrical tilting is also much faster than its mechanical counterpart. The electrical method, however, also has certain drawbacks. In one method, the beam is deflected above the objective lens (pre-lens deflection) in such a way that each ray seems to emerge from a point coincident with the apparent position of the electron source (see FIG. 2). This way, each ray is focussed on the same area of the sample as long as the sample surface is in focus. However, as a consequence, the beam traverses the field of the objective lens considerably off-axis, with attendant degradations due to lens aberrations. In particular, chromatic aberrations limit the attainable resolution to several tens of nanometers. Many applications require a much higher resolution of about 5 nm.
If, as in another method, the deflection coils are arranged below the objective lens (post-lens deflection), the beam passes through the lens on the optical axis (FIG. 3). However, the physical dimensions of the coils below the final lens imposes a limit on the minimum attainable working distance, i.e., on the minimum attainable distance between the final lens and the specimen to be examined. An acceptable resolution is then not achieved due to the degraded instrument resolution arising from the enlarged working distance.
The present invention intends to overcome the above-mentioned drawbacks and disadvantages of the prior art. Specifically, the invention intends to provide an improved charged particle beam column allowing specimens to be examined with a large beam landing angle while maintaining a high resolution of the charged particle image.
Further advantageous features, aspects and details of the invention are evident from the dependent claims, the description and the accompanying drawings. The claims are intended to be understood as a first non-limiting approach to define the invention in general terms.
According to one aspect, the invention provides a column for directing a beam of charged particles onto a specimen surface under a large beam landing angle, the column comprising:
a particle source for providing the beam of charged particles propagating along an optical axis; an objective lens for focussing the beam of charged particles onto the specimen surface; a pre-lens deflection unit arranged between the particle source and the objective lens; the pre-lens deflection unit being adapted to deflect the beam of charged particles away from the optical axis on such a path that the combined action of the objective lens and the pre-lens deflection unit directs the beam of charged particles towards the optical axis to hit the specimen surface from a first direction; an in-lens deflection unit arranged in the vicinity of the objective lens such that the fields of the in-lens deflection unit and the objective lens overlap; the in-lens deflection unit being adapted to redirect the deflected beam of charged particles on such a path that the combined action of the objective lens and the in-lens deflection unit redirects the beam of charged particles towards the optical axis to hit the specimen surface under said large beam landing angle from a second direction substantially opposite to said first direction.
Preferably, the fields of the pre-lens deflection unit and the objective lens have substantially no overlap. It is further advantageous if the in-lens deflection unit and the objective lens have appreciable overlap.
As discussed above, pre-lens deflection leads to an off-axis path of the beam through the objective lens which gives rise to large chromatic aberrations. These chromatic aberrations have been found to be independent of the position of the deflecting system as long as the field of the deflector and the field of the objective lens do not overlap. When the deflection system is placed inside the field of the lens, the chromatic aberrations are reduced. The reduction can amount to 50% or more, if the deflection system is placed deep inside the field of the lens or even partly below the lens. However, the chromatic aberration of such an in-lens deflection system is still in the order of tens of nanometers and thus not acceptable for many applications.
It has surprisingly been found by the present inventors that the chromatic aberrations caused by pre-lens deflection can be compensated by an in-lens deflection in the opposite direction. The combined action of pre-lens deflection and in-lens deflection causes the charged particle beam to hit the sample surface from a direction substantially opposite to the direction from which the beam hits the sample when no in-lens deflection is carried out.
Without being bound to a particular theory, this effect is presently understood as follows: For example, the pre-lens deflection system alone may cause a chromatic aberration of 100 nm for a beam landing angle of 5xc2x0, and the in-lens deflection system alone may cause a chromatic aberration of 50 nm for a beam landing angle of 5xc2x0, i.e. one which is reduced by 50%. Exciting the pre-lens deflection system to tilt the beam by 5xc2x0, and the in-lens deflection system to tilt the beam by 10xc2x0 in the opposite direction then leads to a net tilt angle of 5xc2x0.
The chromatic aberrations amount to 100 nm in both cases, albeit in opposite directions, such that these chromatic aberrations exactly compensate each other. A net beam landing angle of 5xc2x0 at negligible chromatic aberrations results. Of course, the skilled person will appreciate that for different reduction factors of the in-lens system, other relative tilt angles may have to be chosen to achieve compensation. Further, the practice of the invention does not depend on a knowledge of the chromatic aberrations of the deflection systems. The deflection angles of the pre-lens and in-lens deflection system at which for a preselected beam landing angle minimum aberration is obtained may be extracted experimentally from the resulting images.
The invention thus has the advantage that large beam landing angles on the sample surface can be provided without the usual reduction in resolutions arising from large chromatic aberrations.
In a preferred embodiment, the pre-lens deflection unit and the in-lens deflection unit are adapted to provide a beam landing angle less than 25xc2x0, preferably between 3xc2x0 and 15xc2x0, more preferably between 5xc2x0 and 10xc2x0. In the example given above, a beam landing angle of 3xc2x0 would be realized by exciting the pre-lens deflection system to tilt the beam by 3xc2x0, and by exciting the in-lens deflection system to tilt the beam by 6xc2x0 in the opposite direction to achieve a net tilt angle of 3xc2x0.
In a further preferred embodiment, the pre-lens deflection unit comprises two deflectors adapted to deflect the beam of charged particle away from the optical axis to a path seeming to emerge from a point coincident with the apparent position of the particle source or, if applicable, to emerge from a point coincident with the apparent position of an intermediate image of the particle source.
In still a further preferred embodiment, the in-lens deflection unit comprises two deflectors adapted to redirect the deflected beam to cross the optical axis at the specimen surface.
Although the deflection system described so far can be used with any kind of objective lens, in a preferred aspect of the invention, the objective lens is a compound magnetic-electrostatic lens. Preferably, the electrostatic part of the compound magnetic-electrostatic lens is an electrostatic retarding lens. Using such a compound magnetic-electrostatic lens yields superior resolution at low acceleration energies, such as a few hundred electron volts in case of a SEM. Such low acceleration energies are desirable especially in modem semiconductor industry, to avoid charging and/or damaging of radiation sensitive specimens. In a preferred embodiment, the electrostatic retarding lens reduces the energy of a beam of electrons as charged particles to less then 5 keV, more preferably to less then 2 keV, most preferably to about or less than 1 keV.
In a preferred embodiment, the column comprises means for applying a potential difference between the specimen and a pole piece of the objective lens. An electrostatic retarding lens may thus be created between the specimen and a pole piece of the objective lens, without making additional electrodes necessary. The skilled person will appreciate, however, that additional electrodes may be present to supplement and/or modify the thus generated retarding field.
To obtain a sufficient tilt angle, the strength of the deflecting field of the in-lens deflection unit must be larger for an objective lens with a shorter focal length. In some cases this requirement on the in-lens deflection unit may prevent the use of strong objective lenses thereby limiting the achievable resolution of the objective lens in the standard imaging mode, i.e. in a mode where the column is operated without beam tilt.
To overcome this limitation, according to a further preferred aspect of the invention, the objective lens is a two-stage lens, comprising a lower lens with short focal length and an upper lens with larger focal length. If high resolution but no beam tilt is required, only the lower lens would be used. Due to its short focal length, high resolution is achieved. For large beam landing angles, only the upper, weaker lens would be used, resulting in a slightly worse resolution allowing large beam landing angles. As will be appreciated by the skilled person, both lenses may be used simultaneously with relative excitation strengths depending on the required combination of resolution and beam landing angle.
Preferably, the focal length of the lower lens is about or less then 10 mm, more preferably, the focal length of the lower lens is in the range between about 2 mm and about 5 mm. The focal length of the upper lens is preferably about or less than 40 mm, more preferably the focal length of the upper lens is in the range between about 10 mm and about 20 mm. In most cases the focal length of the upper lens will be larger than 3 mm.
In a further preferred embodiment, the in-lens deflection unit is arranged below the upper lens, proximal to the lower lens.
Instead of a two-stage lens with both stages arranged above the specimen, it may be advantageous to put a further strong magnetic lens below the specimen. In a mode with no beam tilt, high resolution may then be achieved. As will be appreciated by the skilled person, also in this case both lenses may be used simultaneously with relative excitation strengths depending on the required combination of resolution and beam landing angle. Also, the additional strong magnetic lens may be combined with a two-stage lens.
Preferably, the column further comprises means for scanning the beam of charged particles over the surface of the specimen.
The invention further comprises a method for directing a beam of charged particles onto a specimen surface under a large beam landing angle, the method comprising the steps of:
a) providing a beam of charged particles propagating along an optical axis;
b) focusing the beam of charged particles onto the specimen surface; whereby the method is characterized in further comprising the steps of
c) selecting a beam landing angle;
d) deflecting the beam of charged particles propagating along the optical axis away from the optical axis,
whereby the magnitude of the deflection is chosen such that the combined action of deflecting and focussing the beam directs the beam of charged particles to hit the specimen surface from a first direction;
the deflection causing a first contribution to the aberration on the specimen surface;
e) redirecting the deflected beam of charged particles,
whereby the redirection is chosen such that the combined action of redirecting and focussing the deflected beam directs the beam of charged particles to his the specimen surface under said large beam landing angle from a second direction substantially opposite to said first direction;
the redirection causing a second contribution to the aberration on the specimen surface;
whereby the deflection in step d) and the redirection in step e) are chosen such that the total aberration on the specimen surface is minimized.
The deflection and redirection can be varied while keeping the beam landing angle constant. That way, the first and second contributions to the aberration can be varied without changing the beam landing angle. In the method according to an embodiment of the invention, the deflection and redirection is then adjusted to yield minimum (total) aberration on the sample surface at constant landing angle. In the framework of the above-mentioned theory this will be the case when the deflection and redirection are such that the first and second contribution to the aberration essentially compensate each other.
According to further aspect, the invention provides a column for directing a beam of charged particles onto a specimen under a large beam landing angle, the column comprising: a particle source for providing a beam of charged particles propagating along an optical axis; an objective lens for focussing the beam of charged particles onto the specimen; a pre-lens deflection unit for deflecting the beam of charged particles away from the optical axis, the pre-lens deflection unit being arranged between the particle source and the objective lens such that the fields of the pre-lens deflection unit and the objective lens have substantially no overlap; an in-lens deflection unit for redirecting the deflected beam of charged particles to the opposite side of the optical axis , the in-lens deflection unit being arranged in the vicinity of the objective lens such that the fields of the in-lens deflection unit and the objective lens have appreciable overlap, such that the combined action of the objective lens and the in-lens deflection unit directs the beam of charged particles back to the optical axis whereby the beam of charged particles hits the specimen under said large beam landing angle.
According to another aspect of the invention, a charged particle column having a defined optical axis is provided, the charged particle column comprising a charged particle source, an objective lens, a pre-lens deflection unit positioned between the source and the objective lens, an in-lens deflection unit proximal to the objective lens and, when activated, creating a deflection field overlapping a focussing field of the objective lens, wherein said pre-lens deflector is operable to deflect charged particles away from the optical axis, and said in-lens deflector is operable to deflect charged particles towards the optical axis.